1. Field of the Invention
The present invention relates to PTC (positive temperature coefficient) thermistors, and their manufacturing methods.
2. Prior Art
PTC (positive temperature coefficient) thermistors are well known devices which have been employed in electronic circuits for over current protection and for thermal sensing. A conventional PTC thermistor is shown in FIG. 17. As can be seen in the illustration, the PTC thermistor SO has a composite structure of sandwiched PTC composition 1a between electrodes 2a and 3a. The above mentioned PTC element 1a is comprised of a PTC composition including polymers and conductive particles which demonstrates positive thermal coefficient resistance properties. The electrodes 2a, 3a are formed from sheet form metallic material, and each is provided with a respective lead 4, 5 connected thereto as shown in FIG. 17.
For the manufacture of this type of PTC thermistor SO, the following method, for instance, can be applied. First of all, as is shown in FIG. 18, two relatively large metallic sheets 2, 3 each of which constitutes a plurality of the individual thermistor electrodes 2a, 3a respectively, are bonded to the opposing upper and lower surfaces of a sheet of PTC composition 1 which is to constitute a plurality of the individual PTC elements 1a, thereby forming a laminated PTC thermistor sheet 6. The above bonding of the metallic sheets 2, 3 to the PTC composition 1 is conventionally achieved using a conductive adhesive agent. Next, as shown in FIG. 19, the PTC thermistor sheet 6 thus fabricated is cut into small thermistor chips 7 of the desired form. Finally, to the both the upper and lower electrode 2a, 3a of each thermistor chip, a respective lead 4, 5 is soldered or spot welded, thereby establishing an electrical connection between lead wire 4, 5 and the electrodes 2a, 3a, whereby the PTC thermistor SO shown in FIG. 17 is fabricated.
With the type of PTC thermistor SO shown in FIG. 17 and for the fabrication method thus described, several problems exist. These problems include the following:
1. It is necessary to prepare the leads 4, 5 from a separate metal sheet or metal wire from that used for the electrodes 2a, 3a. PA1 2. A manufacturing process of connecting the leads 4, 5 to the electrodes 2a, 3a is necessary. PA1 3. Application of heat and pressure to the thermistor chips 7 occurs when the leads 4, 5 are connected by soldering or spot welding. In particular, there is always the possibility that the added heat will deleteriously effect the PTC composition, for example resulting in change in the resistance properties of the composition, deterioration of the composition, weakening of the bond with the electrodes, etc. PA1 4. Variability in the quality of the electrical and physical connection between the leads 4, 5 and the electrodes 2a, 3a is likely to occur which also impairs the performance of the finished thermistor.